Its not that it's harder to, its that it takes time and resources to. Slapping animation data of a male model onto a female model will look really weird. They have to do an entire motion cap set for a whole other playable character. Not to mention the model(s), sounds, etc. It doesnt sound like a lot, but it is.
The other thing to consider is, say they did do that. All the male characters likely share the same sound set. That's x amount of ram for sounds. You'll have to at least double that allocation if a female set is also implemented. Double the animation data too. Since it's going to be multiplayer, they may be already pushing the limits of what they have going already. It could be very possible that adding a second set of data would end up being too much.
All guesswork of course. It's just possible under some circumstances where thats actually true, and they cant implement it.
Though honestly, the fact that theyre all suppose to be Arno makes complete sense as is.